Standard x-ray transmission imaging systems have been used for security, medical, and inspection applications for many decades. Typically, medical transmission x-ray images have been obtained using cone beams of x-rays and utilizing x-ray film as the detection medium. More recently, fan beams of x-rays have been used in conjunction with linear segmented detector arrays to create two dimensional images of objects which are conveyed through the fan beam, between the x-ray source and the detector array. This is also the standard approach for examining small bags or packages for security applications, such as at airports. The transmission image obtained may be a single-energy image, using a single end-point energy x-ray tube and a detector array where each array element consists of only one detector channel.
Images based on differential transmission or scatter as a function of the energy of incident radiation may be obtained using an x-ray source that alternates between two end-point energies, or, alternatively, by employing a dual-energy segmented detector array. Such arrays have two detector channels per array element. One detector channel is sensitive to the lower energy x-rays, while the second channel (which often also contains an x-ray filter) preferentially detects the higher energy x-rays. By taking the ratio of the two signals from the low and high energy channels, dual-energy transmission images can be obtained, which allow the average effective atomic number Z of materials at each location in the image to be determined. This allows materials to be crudely separated into either low-Z (organic), intermediate-Z, or high-Z (metallic) materials. This information can then be overlaid on the black and white transmission image, typically using a color palette, to create a color image which conveys the material identification information to the operator.
Backscatter x-ray imaging has been used in the last couple of decades to provide a means of more reliably detecting and imaging organic materials that have been concealed in bags and packages, and even large cargo containers and vehicles. Instead of using a fan beam of x-rays, these systems typically use a scanning pencil beam of x-rays, also known as a “flying spot”. Backscatter images are created by measuring the amount of x-ray energy that is Compton scattered out of the beam as each part of the object is sequentially illuminated by the beam. The Compton scattered x-rays are typically detected in large-area detectors which have been optimized for detecting the relatively low energy scattered x-rays. By raster scanning the pencil beam across the object being scanned while conveying the object through the scanning beam, a complete two-dimensional backscatter image of the object is obtained. Since the Compton scatter at lower x-ray energies (below about 250 keV) tends to be most sensitive to the organic regions of the object, the method can be used to highlight these regions.
The combination of x-ray transmission and backscatter techniques has previously been taught, for example, in U.S. Pat. No. 6,151,381 (“Gated Transmission and Scatter Detection,” wherein separate and temporally gated sources are used for transmission and backscatter imaging) and U.S. Pat. No. 6,546,072 (“Transmission-Enhanced Scatter Imaging,” wherein the same source is used for both transmission and backscatter images). Both of the foregoing patents are incorporated herein by reference. Systems using both transmission and backscatter imaging have either required identical source spectra for the two modalities (in cases where a single source is used for both) or else have had to contend with cross-talk issues, due especially to the scattered photons from the typically higher-energy or higher-flux transmission fan beam impinging on the scatter detectors.